1. Field of the Invention
The present invention generally relates to a motor-driven power steering control apparatus for controlling a motor-driven power steering system installed on an automobile or motor vehicle or the like. More particularly, the present invention is concerned with an improvement of the motor-driven power steering control apparatus such that steering operation can be performed without bringing about discomfort or offensiveness in the driver of the motor vehicle upon stoppage of a power assist in response to detection of an abnormality or trouble in the control apparatus.
2. Description of Related Art
The conventional motor-driven power steering control apparatus known heretofore includes a steering state detecting means composed of a steering torque detecting means for detecting a steering torque, a steering velocity detecting means for detecting a steering velocity and a vehicle speed detecting means for detecting a vehicle seed. Further, the control apparatus includes a motor drive command value arithmetic means for arithmetically determining a motor drive command value on the basis of a detection signal outputted from the steering state detecting means, a motor driving means for driving an assist motor of the steering system on the basis of the motor drive command value, a trouble detecting means for detecting a trouble of the control apparatus to output a trouble detection signal and a motor drive command value correcting means for decrementing as a function of time lapse the above-mentioned motor drive command value on the basis of the detection signal outputted from the trouble detecting means. (For more particulars, reference may have to be made to Japanese Patent Publication No. 94227/1995 and Japanese Patent Application Laid-Open Publication No. 81590/1995 (JP-A-1995-81590)).
In the conventional motor-driven power steering control apparatus mentioned above, the motor drive command value is not instantaneously set to zero when a trouble of the control apparatus is detected but the motor drive command value having been effective immediately before the occurrence of trouble is set as an initial value which is then gradually decreased or decremented as a function of time lapse to thereby suppress raid change of the steering torque upon occurrence of the trouble so that the driver of the motor vehicle can get free of the feeling of discomfort in his or her steering operation.
The conventional motor-driven power steering control apparatus mentioned above suffers a problem that when occurrence of a trouble is detected in the control apparatus at the moment the driver turns quickly or steeply the steering wheel or the moment a wheel or wheels of the motor vehicle drop into a rut in a road in the course of running, the motor drive command value increased instantaneously is stored as the initial value to be decremented from which gradual decreasing or decrementing of the motor drive command value is started, as a result of which the driver will feel discomfort in the steering operation because it becomes light or heavy as compared with the ordinary steering.
In addition, when the driver reverses the steering direction in the course of decrementing the motor drive command value after detection of trouble in the control apparatus, the motor torque is imposed as a load on the driver, as a result of which the steering operation (i.e., manipulation of the steering wheel) becomes heavier, giving rise to a problem.